Environment monitoring systems on vehicles are used to determine objects in the vehicle environment. In the case of rear area monitoring systems, the intention is to determine, in particular, a possible collision with objects in the rear area (environment behind the vehicle) during reversing.
For this purpose, the environment monitoring systems have distance sensors. During propagation time measurements, a distance sensor emits a detection signal into the region to be monitored at an emission time. If an object is detected by the detection signal, the object reflects back the signal, with the result that the distance sensor can detect it at a receiving time. The propagation time of the detection signal can be determined as the difference between the receiving time and the emission time, with the result that the total distance, which represents twice the distance between the object and the sensor, can be determined using the signal speed. Such propagation time measurements are carried out, in particular, with ultrasonic sensors and RADAR sensors, to some extent even with light beams (LASER) as the detection signals. Direction-dependent or angle-resolved detection cannot be carried out with propagation time measurements for the time being.
DE 10 2007 052 977 A1, for example, proposes triangulation in which two ultrasonic sensors are arranged in a horizontal line in the bumper region of a vehicle and each separately carry out a propagation time measurement, with the result that two items of distance information are determined and can be used to determine the distance between the sensor and the vehicle or the environment monitoring system by means of triangulation, such a distance generally being determined as the minimum distance from the vehicle. DE 10 2006 002 232 B4 also proposes such triangulation for determining the position of an object by measuring two distances from two different positions.
During such triangulation processes, a triangle can thus be determined given the known sensor distance (distance between the sensors) and the separately determined individual distances between an object and each of two distance sensors, with the result that the distance between the object and the monitoring system is determined as the height in this triangle. DE 10 2007 042 220 A1 also proposes such triangulation using ultrasonic sensors. DE 41 37 068 A1 provides an integrated optical multiple distance sensor and proposes optical triangulation using position-sensitive diodes.
DE 195 07 957 C1 proposes triangulation using infrared LEDs, a road surface being scanned in order to detect a lane boundary. DE 102 51 357 A1 discloses a method for setting or switching off a travel direction indicator, in which lane and/or travel direction changes are determined from the environmental data; a distance measurement as a propagation time measurement using infrared sensors of a mono camera and using triangulation of a stereo camera is also described in addition to lane detection.
Such triangulation methods presuppose that each distance sensor detects the object to be determined at substantially the same location and a triangle is thus formed. However, such determination by means of triangulation may be more complex in the case of larger objects. Furthermore, one of the distance sensors may possibly also not detect a measurement signal if, for example, the object has oblique surfaces running in an unfavourable manner, since RADAR beams and also ultrasonic waves undergo directed reflection, with reflections on unfavourable oblique planes possibly not resulting in an echo at the distance sensor. The large number of distance sensors, usually six to eight distance sensors for a vehicle width of a commercial vehicle of 2.5 m, for example, which is generally required for conventional rear area monitoring systems is also disadvantageous.